Metal tubes are subjected to the cold working process when quality, strength and dimensional accuracy in as-hot-rolled condition are not satisfied. In the cold working process, there are available the cold drawing method using a solid die and a plug or a mandrel and the cold rolling method by a cold Pilger mill, and the present invention relates to the cold drawing method.
In the cold drawing method, an end portion of a hollow shell is pointed by a point squeezing machine, surface scale is removed by pickling, lubrication treatment is then performed, and drawing is performed by passing the hollow shell through a die. The cold drawing method is classified as plug drawing, floating plug drawing, mandrel drawing and sinking, and all of them are performed by diameter-reducing working with the aid of dies.
FIG. 1 is a diagram to explain a conventional diameter-reducing drawing method and shows the case of plug drawing. The plug drawing shown in this figure is the most frequently used drawing method, which involves inserting a plug 3 with a plug supporting bar 4 into a hollow shell 1, chucking the pointed end of the hollow shell 1 by use of a chuck 5, passing the hollow shell 1 through a die 2, and drawing the hollow shell 1 in the direction of arrow shown by symbol X in the figure. This method is excellent also in plug exchange and workability and enables high reduction rate to be applied.
In case of mandrel drawing whose explanatory diagram is omitted, a mandrel is inserted into a hollow shell, and the hollow shell is drawn by being passed through a die in the same way as described in FIG. 1 above. High dimensional accuracy and good inner surface quality can be ensured by this method for small-diameter tubes, because the working of inner surfaces of tubes is performed by use of a mandrel. This method is therefore used in the manufacture of high-grade tubes for nuclear and other applications.
As for drawing machines used in cold drawing, those of the chain type by motor driving are widely adopted. Oil-hydraulic and water-hydraulic ones are also used.
In the cold drawing process of metal tubes, frictional resistance is generated between the outer surface of a tube material and the die surface, and between the inner surface of a tube material and the plug or the mandrel surface, and drawing work is performed against the frictional resistance. Therefore, tension is generated in the tube material in a longitudinal direction thereof. Drawing-induced-slenderizing in diameter begins when tensile stress, given by dividing this tension by the cross-sectional area of drawn tube material, increases, and the tube material is to be torn off when tensile stress reaches the deformation resistance of the tube material.
Because the thinner the wall thickness of tube is, the larger the tensile stress in a longitudinal direction of tube tends to be and the more likely the tube is broken off, there is a limit to the wall thickness reduction rate. Therefore, in drawing work which requires a critical increase in thickness reduction rate, it is necessary to repeat drawing work by increasing passes/times of drawing and lubrication treatment becomes necessary whenever the drawing is repeated, thus ending up in high costs. When the work hardening of a tube material is noticeable, annealing work may also be necessary between repeated drawing steps.
Citation List
Patent Literature
Patent Literature 1: PCT/JP2008/051619